1. Field of the Invention
This invention relates to the field of electrospinning fibers from polymer solutions.
2. Background of the Invention
Nanofibers are useful in a variety of fields from clothing industry to military applications. For example, in the biomaterial field, there is a strong interest in developing structures based on nanofibers that provide a scaffolding for tissue growth effectively supporting living cells. In the textile field, there is a strong interest in nanofibers because the nanofibers have a high surface area per unit mass that provides light but highly wear-resistant garments. As a class, carbon nanofibers are being used for example in reinforced composites, in heat management, and in reinforcement of elastomers. Many potential applications for nanofibers are being developed as the ability to manufacture and control the chemical and physical properties improves.
Electrospray/electrospinning techniques can be used to form particles and fibers as small as one nanometer in a principal direction. The phenomenon of electrospray involves the formation of a droplet of polymer melt at an end of a needle, the electric charging of that droplet, and an expulsion of parts of the droplet because of the repulsive electric force due to the electric charges. In electrospraying, a solvent present in the parts of the droplet evaporates and small particles are formed but not fibers. The electrospinning technique is similar to the electrospray technique. However, in electrospinning and during the expulsion, fibers are formed from the liquid as the parts are expelled.
Glass fibers have existed in a sub-micron range for some time. Small micron diameter fibers have been manufactured and used commercially for air filtration applications for more than twenty years. Polymeric melt blown fibers have more recently been produced with diameters less than a micron. Several value-added nonwoven applications, including filtration, barrier fabrics, wipes, personal care, medical and pharmaceutical applications may benefit from the interesting technical properties of nanofibers and nanofiber webs. Electrospun nanofibers have a dimension less than 1 μm in one direction and preferably a dimension less than 100 nm in this direction. Nanofiber webs have typically been applied onto various substrates selected to provide appropriate mechanical properties and to provide complementary functionality to the nanofiber web. In the case of nanofiber filter media, substrates have been selected for pleating, filter fabrication, durability in use, and filter cleaning considerations.
A basic electrospinning apparatus 10 is shown in FIG. 1 for the production of nanofibers. The apparatus 10 produces an electric field 12 that guides a polymer melt or solution 14 extruded from a tip 16 of a needle 18 to an exterior electrode 20. An enclosure/syringe 22 stores the polymer solution 14. Conventionally, one end of a voltage source HV is electrically connected directly to the needle 18, and the other end of the voltage source HV is electrically connected to the exterior electrode 20. The electric field 12 created between the tip 16 and the exterior electrode 20 causes the polymer solution 14 to overcome cohesive forces that hold the polymer solution together. A jet of the polymer is drawn by the electric field 12 from the tip 16 toward the exterior electrode 20 (i.e. electric field extracted), and dries during flight from the needle 18 to the exterior electrode 20 to form polymeric fibers. The fibers are typically collected downstream on the exterior electrode 20.
The electrospinning process has been documented using a variety of polymers. One process of forming nanofibers is described for example in Structure Formation in Polymeric Fibers, by D. Salem, Hanser Publishers, 2001, the entire contents of which are incorporated herein by reference. By choosing a suitable polymer and solvent system, nanofibers with diameters less than 1 micron have been made.
Examples of fluids suitable for electrospraying and electrospinning include molten pitch, polymer solutions, polymer melts, polymers that are precursors to ceramics, and/or molten glassy materials. The polymers can include nylon, fluoropolymers, polyolefins, polyimides, polyesters, and other engineering polymers or textile forming polymers. A variety of fluids or materials besides those listed above have been used to make fibers including pure liquids, solutions of fibers, mixtures with small particles and biological polymers. A review and a list of the materials used to make fibers are described in U.S. Patent Application Publications 2002/0090725 A1 and 2002/0100725 A1, and in Huang et al., Composites Science and Technology, vol. 63, 2003, the entire contents of which are incorporated herein by reference. U.S. Patent Appl. Publication No. 2002/0090725 A1 describes biological materials and bio-compatible materials to be electroprocessed, as well as solvents that can be used for these materials. U.S. Patent Appl. Publication No. 2002/0100725 A1 describes, besides the solvents and materials used for nanofibers, the difficulties of large scale production of the nanofibers including the volatilization of solvents in small spaces. Huang et al. give a partial list of materials/solvents that can be used to produce the nanofibers.
Despite the advances in the art, the application of nano-fibers has been limited due to the narrow range of processing conditions over which the nano-fibers can be produced. Excursions either stop the electrospinning process or produce particles of electrosprayed material.